When manufacturing the panels of the vehicle body, a draw forming (or a drawing) is performed as the press molding. Popularly in the draw forming, a press molding apparatus is used, having a male mold called a punch, a female mold called a die arranged to face the male mold and a mold called a cushion for clamping a material in cooperated with the female mold. Using such apparatus, a sheet material such as a sheet metal (hereinafter called “blank”) is formed into the desired shape. In draw forming, the periphery of the blank is clamped by clamping faces and the male and female molds approach each other, so that the blank is pressed and drawn in the molds. In the molding process, the approaching of the male and female molds causes the plastic deformation of the blank following the shape of molding surfaces, and the blank flows in the predetermined direction from the clamped portions.
The draw forming has problems such as a crack (break) in the blank. The cracks occurred in the molding are caused by the depth of drawing, the inflow of the blank in molding or the like. The conventional technique to prevent the cracks occurred in drawing is disclosed, for example, in JP S62-142033 A. JP S62-142033 A discloses the method that the angle of the clamping faces (“clamping angle”) for clamping the blank is inclined by the predetermined angle to provide easy flow of the blank.
The technique disclosed in JP S62-142033 A (hereinafter called “conventional technique”) makes the inflow of the blank smooth, and prevents the cracks in drawing. However, the conventional technique has problems as follows.
The molded articles molded by the draw forming include the panels of the vehicle body, which have complex curved shapes (hereinafter called “complex shaped articles”). In draw forming of the complex shaped articles, which have asymmetrical shapes, the forming amount of the blank differs locally (namely, the depth of drawing is not even).
Therefore, when employing the conventional technique for molding the complex shaped article, the larger clamping angle has to be set for the portion where the forming amount is comparatively large. In the case that the clamping angle becomes too large, there occurs a shortage of the clamping force. The shortage causes wrinkles in drawing.
Furthermore, in the molded article, the clamped portion may be used as a part of the product such as a flange. In draw forming such article, there is a restriction depending on the product shape, so that it is difficult to employ the conventional technique.
As mentioned above, in the case that the molded article has a simple shape and the blank flows from all around the clamped periphery, the conventional technique works well. On the other hand, in the case that the article has the complex shape or that the clamped periphery is used as a part of the product, the conventional technique does not work well.